An error popup has started in Thunderbird. I have my Google calendar synced to Thunderbird on my Win10 computer. Says "this item has recently been changed on the server"-----see attached image. If I click "Discard my changes and reload" it does nothing. If I click "Delete anyway" the error popup remains. I have to go to Task Manager to close Thunderbird and restart where the error pops up again. The calendar item it describes is no longer on my calendar. Ideas on making the popup stop?